#app4{}
@keyframes change {
    0%{background: red}
    100%{background: blue}
}
#app4 .circle{
    width: 10vw;
    height: 10vw;
    border-radius: 50%;
    border: 1px solid red;

}
/*同生死共存亡，缺一不可*/
#app4 .circle.active{
    animation: change 1s infinite alternate linear;
}